Disk recorders of either the magnetic or optical type employ transducer positioning systems. Such positioning systems often use a so-called velocity servo loop for long transducer motions, termed seeks. Such long transducer motions cause a sensing transducer to traverse a large number of concentric circular record tracks on the disk record member. The velocity positioning servo loop is optimally switched to a track-following position servo mode at a one-quarter track pitch distance from a target track. Such a track following control may be favorably compared to "Stop-Lock" positioning control in other servo positioning applications. Upon reaching the target track, the track-following position servo positions the transducer to faithfully scan or follow the target track. In a subsequent seek operation, the track following position loop is interrupted to return to either a velocity loop, a second positioning loop or other form of seek positioning servo control. It has been observed that when inter-track spacing is reduced for obtaining higher track densities, then reliable track counting during a velocity seek becomes more acute. In particular, the so-called runout or eccentricity of rotation of the optical disk can cause false direction indications and false counting of tracks such that a target track is not faithfully reached. Accordingly, it is desired to provide for more reliable counting and control of the velocity servo during a seek operation in which a transducer transverses a plurality of tracks.
In a seek operation, because of the high track densities, any asperity in the record medium surface can cause a track not to be counted. Accordingly, it is desired to provide a system which obviates medium surface asperities from detracting from a successful track-seeking operation.
Many optical recorders have the goal of high performance at low cost. In some of these high performance optical recorders, a so-called fine servo or fine actuator (also termed a secondary head-arm) is carried on a primary transducer carrying-arm which is radially moved by a so-called coarse actuator. Typically, the fine actuator has high-frequency response characteristics and provides for rapid and short distance positioning of the transducer with respect to a track being followed or for moving from one track to a second or target track. The coarse servo, which positions the relatively large mass primary or head-carrying head-arm, typically has low-frequency characteristics for handling the longer moves. For optimizing the relationship for top performance between the fine and coarse actuators, servo systems also provide for relative positioning of the fine actuator with respect to the coarse actuator to a central or reference position. Such arrangements have been colloquially called "piggy-back" carriage servo systems. All of the above shows that the need for reliable track counting becomes more acute.